1. Field of the Invention
The present invention relates to a rotary atomizing electrostatic coating apparatus, and more particularly to a rotary atomizing electrostatic coating apparatus having a structure capable of increasing uniformly discharge current formed in a front area portion of rotary shaft of a spray head.
2. Description of the Prior Art
An electrostatic coating method is a method of using a grounded object as anode and coating apparatus as cathode, applying a high negative voltage thereto to form an electrostatic field between the two electrodes, charging sprayed paint particles negatively, and adhering the paint efficiently on the object of the opposite polarity. Setting of the object as anode and painting apparatus as cathode is based on the practical safety precaution that spark discharge hardly occurs as compared with reverse polarity.
The electrostatic coating apparatus may be roughly classified into two types: one is a rotary atomizing electrostatic coating apparatus for pulverizing the paint by the force of rotation and static electricity, and adhering paint particles on the object by electrostatic attraction, and other is a gun type apparatus for pulverizing the paint by the force of compressed air, or high paint pressure or mechanical force, and charging the pulverized paint electrically and adhering to the object by electrostatic attraction.
The invention relates to the former type of rotary atomizing electrostatic coating apparatus. In the rotary atomizing electrostatic apparatus, generally, the paint is atomized by centrifugal force of spray head such as rotating metallic cup (or "bell"), disk or the like, and a high voltage is applied to such metallic spray head in order to charge the paint electrically and form an electrostatic field. Because this cup has relatively excellent characteristics satisfying functions of atomizing the paint, and maintaining an electrostatic field opposite to the object, simultaneously, the rotary atomizing electrostatic coating apparatus is known as a coating apparatus exhibiting a high paint deposition efficiency.
More specifically, the leading end part of a conventional rotary atomizing electrostatic coating apparatus comprises a metal cup as a spray head fixed to the leading end of a hollow shaft of a motor incorporated inside the apparatus. Inside the hollow shaft, a paint nozzle communicating with a paint pump is disposed coaxially. The top of the paint nozzle reaches up to the inside of the metal cup, and communicates with paint outlet holes formed around the central position of the front side of the cup (multiple pores are disposed annularly as the paint outlet holes). At the rear side of the cup, an air ring is provided coaxially to enclose the outer circumference of the hollow shaft at a specific spacing--inside the air ring are formed many air outlet holes opened towards the rear side of the cup. The air outlet holes communicate to an air feed pump through an air lead-in port opening located at the side face of the air ring. The hollow shaft is electrically connected to a high voltage generator through cables, and a specific voltage can be applied to the cup communicating with the hollow shaft.
In this apparatus, the paint fed through rear end of the hollow shaft is supplied into the front plane of the metal cup through the paint outlet holes. Since this metal cup as spray head is rotated at high speed by the motor, the paint supplied to the front plane of the metal cup is drawn thinly along the front plane formed on the front side of the metal cup by the centrifugal force, and comes opposite to the outer peripheral edge of the metal cup, and is released in atomized form through the outer peripheral edge of the metal cup. The released paint particles are controlled into a specific pattern by pattern adjusting air ejected from the air outlet holes of the air ring disposed at the rear side of the cup, and carried into the direction of the object. On the other hand, since a high voltage is applied to the metal cup, corona discharge is occurred from the leading end of the metal cup toward the object. The paint particles released from the metal cup are further charged by the corona discharge, aside from the electric charge obtained by contacting with the metal cup, and thus charged paint particles are adhered to the object efficiently by Coulomb's force.
In such conventional rotary atomizing electrostatic spraying apparatus, hitherto, the spray head is a metal cup made of high conductivity material such as chrome steel, stainless steel, and aluminum alloy, as mentioned above. Since the metal cup function as discharge electrode, at a glance, these metals are appeared to be a rational materials.
As a result of investigations by the present inventor, however, in the rotary atomizing electrostatic coating apparatus having such metallic spray head, the discharge. current cannot be adjusted, and it was found difficult to enhance the paint deposition efficiency further. That is, in the conventional rotary atomizing electrostatic coating apparatus having such metallic spray head, atomization of paint was found to cause the reduction of the corona discharge current. This is considered because the atomized paint particles have an electrostatic shielding effect.